Silver nano-particles can be sintered even at a low temperature. Utilizing this property, a silver coating composition containing silver nano-particles is used to form electrodes or conductive circuit patterns on a substrate in production of various electronic devices. Silver nano-particles are usually dispersed in an organic solvent. Silver nano-particles have an average primary particle diameter of about several nanometers to about several tens of nanometers, and their surfaces are usually coated with an organic stabilizer (protective agent). When the substrate is a plastic film or sheet, silver nano-particles need to be sintered at a low temperature (e.g., at 200° C. or less) less than a heat resistant temperature of the plastic substrate.
Particularly, attempts have been recently made to form fine metal lines (e.g., silver lines) not only on heat-resistant polyimide substrates that are already in use as substrates for flexible printed circuit boards but also on substrates made of various plastics, such as PET (polyethylene terephthalate) and polypropylene, that have lower heat resistance than polyimide but can be easily processed and are cheap. When plastic substrates having low heat resistance are used, metal nano-particles (e.g., silver nano-particles) need to be sintered at a lower temperature.
Silver nano-particles have an average primary particle diameter of about several nanometers to about several tens of nanometers, and are more likely to agglomerate than micron (μm)-size particles. Therefore, the reduction reaction of a silver compound (thermal decomposition reaction of a silver oxalate complex compound) is performed in the presence of an organic stabilizer (protective agent such as an aliphatic amine or an aliphatic carboxylic acid) so that the surfaces of resulting silver nano-particles are coated with the organic stabilizer.
Meanwhile, silver nano-particles are used in a silver coating composition (silver ink or silver paste) in which the particles are contained in an organic solvent. In order to development conductivity, an organic stabilizer coating the silver nano-particles needs to be removed during calcining performed after application of the silver coating composition onto a substrate to sinter the silver particles. When the temperature of the calcining is low, the organic stabilizer is poorly removed. When the silver particles are not sufficiently sintered, a low resistance value cannot be achieved. That is, the organic stabilizer present on the surfaces of the silver nano-particles contributes to the stabilization of the silver nano-particles, but on the other hand, interferes with the sintering of the silver nano-particles (especially, sintering by low-temperature calcining).
The use of an aliphatic amine compound and/or an aliphatic carboxylic acid compound each having a relatively long chain (e.g., 8 or more carbon atoms) as an organic stabilizer makes it easy to stabilize silver nano-particles because it is easy to ensure space between the silver nano-particles. On the other hand, the long-chain aliphatic amine compound and/or the long-chain aliphatic carboxylic acid compound are/is poorly removed when the temperature of calcining is low.
As described above, the relationship between the stabilization of silver nano-particles and the development of a low resistance value by low-temperature calcining is a trade-off.
For example, JP-A-2008-214695 discloses a method for producing silver ultrafine particles, comprising reacting silver oxalate and oleylamine to form a complex compound containing at least silver, oleylamine and an oxalate ion; and thermally decomposing the formed complex compound to form silver ultrafine particles (claim 1). Further, JP-A-2008-214695 discloses that in the above method, a saturated aliphatic amine having 1 to 18 carbon atoms in total is reacted in addition to the silver oxalate and the oleylamine (claims 2 and 3), so that a complex compound can be easily formed, the time required to produce silver ultrafine particles can be reduced, and the silver ultrafine particles protected by these amines can be formed in higher yield (paragraph [0011]). Further, the same document discloses that a solvent such as methanol, ethanol, or water may be added when forming a complex compound (paragraph [0017]).
JP-A-2010-265543 discloses a method for producing coated silver ultrafine particles, comprising the first step of mixing a silver compound that is decomposed by heating to generate metallic silver, a mid- to short-chain alkylamine having a boiling point of 100° C. to 250° C., and a mid- to short-chain alkyldiamine having a boiling point of 100° C. to 250° C. to prepare a complex compound containing the silver compound, the alkylamine and the alkyldiamine; and the second step of thermally decomposing the complex compound (claim 3, paragraphs [0061] and [0062]). Further, the same document discloses that there are no restrictions on using a reaction solvent such as methanol, or water to form a complex compound (paragraph [0068]).